Thermal stress inside a disabled submarine

Abstract

Background

Submarine operations require strict adherence to standard operating and safety procedures and errors in judgement or accidents could lead to catastrophe and impair the submarine's ability to surface. In case of disablement of a submarine (DISSUB), the crew would have to survive inside the submarine for a variable period awaiting rescue. Microclimate and habitability of the submarine would have to be maintained and crew would have to consume emergency rations and water.

Methods

In order to validate these procedures, a simulation was carried out in which 80 crew members were closed up inside a submarine in harbour for 24 h simulating a DISSUB situation without power and ventilation.

Results

Average temperature of the submarine compartments rose from 29.33 °C at the beginning of the simulation to 33.5 °C at the end of 24 h. Relative humidity increased from 79% to 87.67%. Crew members consumed an average to 973 kcal worth of rations during the 24 h of the exercise with 500 ml water.

Conclusion

Submarine crew could survive successfully inside a disabled submarine awaiting rescue if thermal stress could be addressed. In the present simulation, the crew suffered from effects of thermal stress. Thermal stress would not only affect damage control capabilities, but could also lead the crew into earlier escape. Greater research and further studies are required to mitigate thermal stress and its effects in order to prolong survival.

Introduction

Submarine operations require strict adherence to standard operating and safety procedures and errors in judgement or accidents could lead to catastrophe and impair the submarine's ability to surface. In such case, the submarine would settle to the bottom of the continental shelf and will be termed as a Disabled Submarine (DISSUB). The crew of the submarine would have an option to either escape on their own using the submarine escape set provided onboard or wait for rescue by assistance from surface forces. Escape is fraught with the risk of decompression sickness, pulmonary barotrauma and various other medical problems and hence rescue by surface forces is preferred. Usual time for surface forces to arrive at the location of the submarine and commence rescue operations could vary from 3 to 7 days.^{1, 2} This phase where the crew awaits rescue is known as the survival phase. During this phase, the crew would survive on the emergency rations and water provided onboard while oxygen and carbon dioxide would be maintained using the submarine's Atmosphere Regeneration System.

Survival simulation ‘exercises’ onboard submarines are routinely carried out by Navies of many countries to evaluate habitability conditions, efficiency of the regeneration systems, procedures for damage control and physiological effects of the conditions on crew as well as to test the command and control organisation during a DISSUB.^{1, 3, 4} In accordance with the experience of Squalus in 1939, it was believed that crew inside a disabled submarine would be hypothermic⁵ and that the internal temperature of the submarine would equalise rapidly with ambient temperature. This had then led the US and Royal Navy to conduct simulated survival experiments inside environmental chambers simulating cold conditions.^{2, 6} However, experience with conducting survival simulations onboard submarines proved otherwise. Simulations by the Norwegian, French and Swedish Navies in bottomed submarines for 3–6 days showed that the temperature within the ‘disabled’ submarines did not fall below 13–14 °C despite ambient sea water temperature of 4–8 °C.¹ US Navy conducted two survival simulations onboard submarines in harbour in 2003 and 2004. In the 2003 simulation with a crew strength of 94, internal temperature of the submarine rose from 21 to 26 °C with ambient air temperature of 5 °C and water temperature of 2.7 °C.³ The simulation in 2004 also faced issues with thermal stress and was terminated prematurely. The underlying finding amongst all these simulations was that the internal temperature of the submarine remained hotter than ambient water temperature which ranged from 4 to 8 °C. While this may in fact be beneficial in some parts of the world, it means that the internal temperature of the submarine in the tropical waters of the Indian subcontinent would be expected to be even higher due to higher water temperatures. This was proven in part by a survival simulation conducted by the Republic of Singapore onboard a submarine in harbour.⁴ All 33 crew members taking part in the simulation suffered heat stress and water loss to the tune of 8.9 L per man in 48 h.

Indian Navy carried out its first survival simulation ‘exercise’ for 24 h in harbour. The aim of the simulation was to validate existing procedures for survival inside the submarine, determine the microclimate and temperature profile along with any effects on crew. This paper provides a summary of the findings of the simulation in terms of methodology followed, habitability conditions seen onboard and a discussion on the medical aspects of the simulation.

Material and methods

During the simulation, 80 submarine personnel were closed up inside a submarine in harbour for 24 h simulating a DISSUB situation. All power equipment was shut down, ventilation was stopped and only emergency lighting was left available. Microclimate was maintained using the atmosphere regeneration system. Levels of oxygen and carbon dioxide along with temperature and relative humidity (RH) were measured hourly in each compartment using portable gas analysers and dry and wet bulb thermometers respectively. The crew was provided with rations as per the authorised scale of emergency rations. These rations consisted of various locally available food items (weighing a total of 410 g) along with 500 ml water per person per day. The authorisation of water was in accordance with usual sea survival rations onboard life rafts and in personal survival packs. Crew wore standard issue cotton clothing which did not add to any thermal discomfort during the simulation. All crew when not on watch-keeping duties were advised complete rest in accordance with standard submarine survival procedures in order to reduce carbon dioxide production, oxygen consumption as well as reduce metabolic body heat.

A pre- and post-simulation medical examination of crew was carried out which included a clinical examination, including heart rate, blood pressure and weight; urine samples for routine examination and specific gravity, and blood samples for urea, creatinine, sodium and potassium measured using a semi-auto analyser (Microlab, Biochemistry Semi-Auto-Analyser ARX-199). In addition, random blood sugar using dip sticks was also checked during the post-simulation medical examination.

During the simulation, all crew members were instructed to pass urine in a measuring flask, record the output and then dispose the urine. Sweat output was calculated from changes in body weight adjusted for fluid intake, food intake and urine output. Plasma osmolality, post-simulation, was calculated from serum sodium, glucose and urea levels. Pre-simulation plasma osmolality could not be calculated because pre-simulation glucose levels had not been measured.

All crew members were informed in detail about the simulation requirements and procedures, crew duties, microclimate monitoring techniques and medical examination requirements subsequent to which consent was obtained. Those with any pre-existing illness were excluded from the simulation. Data was collected, tabulated and analysed using MS Excel 2010 and paired t-test was used for statistical analysis. p-Value was set at 0.05 for significance.

Results

As planned, 80 male volunteers participated in the simulation. Mean age of the participants was 29.48 years (SD 5.81). Average temperature of the submarine rose from 29.33 °C (ranging from 27 to 32 °C in the submarine compartments) at the beginning of the simulation to 33.5 °C (ranging from 32 to 35 °C in the submarine compartments). RH increased from 79% (range 76–85%) to 87.67% (range 83–93%) (Fig. 1). Maximum carbon dioxide level was recorded at 1.1% and minimum oxygen level at 19.6%; both parameters remained well within the targeted levels of 1.3% and 18.5% respectively.

Fig. 1.

Mean temperature and humidity measurements of all compartments.

Crew members consumed an average to 973 kcal (net weight 410 g) worth of rations during the 24 h of the simulation with 500 ml water. All crew lost weight during the simulation. Mean weight loss was 1.51 kg (2.19% of body weight) (p < 0.001) (Table 1).

Table 1.

Results of clinical examination before and after the simulation.

Parameters	Pre-simulation	Post-simulation	p-value
Weight (kg)	68.84 ± 6.71	67.73 ± 6.60	0.000
Heart rate (beats/min)	78.62 ± 10.74	85.52 ± 11.17	0.000
Systolic BP (mmHg)	125.63 ± 9.97	120.19 ± 11.09	0.000
Diastolic BP (mmHg)	71.32 ± 8.71	74.28 ± 9.02	0.001

All values given as Mean ± SD.

After the termination of simulation, it was seen that two individuals did not record their urine output. Excluding these individuals, mean total urine output was seen to be 800 ml (n = 78, SD = 390). Twenty individuals had urine output less than 500 ml in 24 h. Sweat output was calculated from changes in body weight considering that the only intake was 410 g of food and 500 ml of water and the only output was urine and sweat apart from insensible respiratory water loss. Mean sweat output (n = 78) was calculated to be 1570 ml in 24 h. A strong and significant correlation was seen between weight loss and sweat output (r² = 0.72; p = 0.000).

Systolic blood pressure increased in 24 individuals, remained unchanged in three and decreased in rest of the 53 individuals. Diastolic blood pressure increased in 47 individuals, remained unchanged in one and decreased in 32 individuals. Overall average systolic blood pressure of the crew decreased and diastolic pressure and pulse rate increased significantly during the simulation (Table 1). There was a significant increase in mean blood urea and serum creatinine levels during the simulation (Table 2) even though the individual values remained within normal limits. No significant change was seen in urine specific gravity. Mean post-simulation plasma osmolality was calculated to be 305.8 mOsm/kg. Fifty-eight individuals had plasma osmolality more than 300 mOsm/kg and the highest recorded plasma osmolality was 328.05 mOsm/kg. A significant decrease in serum sodium and potassium levels was also seen in the post-simulation samples compared to pre-simulation levels.

Table 2.

Mean laboratory parameters before and after the simulation.

Parameters	Pre-simulation	Post-simulation	p-value
B. urea (mg/dl)	27.86 ± 2.25	29.14 ± 4.04	0.002
S. creatinine (mg/dl)	1.01 ± 0.10	1.15 ± 0.17	0.000
Urine specific gravity	1.01 ± 0.0	1.01 ± 0.0	NS
S. sodium (mEq/L)	137.07 ± 4.01	135.52 ± 3.10	0.000
S. potassium (mEq/L)	4.27 ± 0.42	4.08 ± 0.42	0.000

All values given as Mean ± SD.

Discussion

In tune with the previous simulations carried out by Navies across the world, the major finding of this simulation was that the internal temperature of the submarine remained higher than ambient water temperature.^{7, 8} During the present simulation, submarine's internal temperature increased to 35 °C when the ambient air temperature was 30 °C and surface sea water temperature was 27 °C. RH too increased steadily in all compartments. The state inside the submarine therefore, was that of a high temperature, high humidity and zero air movement (due to shut down ventilation system), a state which is known as that of uncompensable thermal stress.^{9, 10, 11} Heat sources that contributed to high temperature could be metabolic heat from the crew and the highly exothermic atmospheric regeneration system. High RH too would have contributed to thermal stress by reducing evaporative cooling to almost nil.^{11, 12}

The weight loss in crew, fall in systolic BP, high serum osmolality, low urine output in 20 crew members and electrolyte changes, all indicate that thermal stress did affect the crew during the 24 h of the simulation. Tolerance to thermal stress is affected to a large extent by the hydration status at the time of commencement of heat stress and fluid replenishment during the period of heat stress.^{9, 10} It has been reported that adequate fluid replacement during the period of heat stress can even act as a replacement for heat acclimatisation towards increasing thermal tolerance.⁹ Considering that the crew were allowed fluid consumption ad libitum before the simulation commenced, it may be assumed that a state of hypo-hydration did not exist at the time of commencement of the simulation. However, during the 24 h of the simulation, crew consumed only 500 ml water per person and lack of adequate replenishment of water with resultant dehydration could have been a major contributor to the thermal stress.

Lack of hydration during thermal stress can lead to significant cardiovascular strain particularly if the individual is exercising leading to reduced cardiac output, reduced stroke volume and reduced arterial pressure.¹³ Inadequate hydration has been seen to act independent of environmental heat stress in worsening the cardiovascular strain and even marginal water deficit (approximately 1–2% body weight loss) reduces physical work capacity.¹⁴ The results of the present simulation show that the crew did face water deficit (mean weight loss of 2.19% of body weight) as well as cardiovascular strain (increased heart rate, reduced systolic BP, increased diastolic BP, reduced pulse pressure).

It is well known that salt content of sweat is high in un-acclimatised individuals and salt loss in sweat is reduced in trained fit individuals and with heat acclimatisation. It has been reported that in conditions of thermal stress, except during the first few days of heat exposure, salt supplementation may not be required as dietary intake would cover most of the losses.¹⁴ In the present simulation, dietary intake of salt was low as most items in the emergency rations were sweet and did not contain salt. In addition, neither the heat acclimatisation status of the crew was known nor was the salt content of sweat measured. There was however, a measurable reduction in the serum sodium concentration for the crew. It may be likely then, that lack of dietary salt could also have contributed to strain as a result of thermal stress.

The findings of the present simulation in terms of the temperature and humidity profile and the thermal stress have important repercussions for the crew in case of an actual DISSUB scenario. In an actual DISSUB, damage to certain compartments would result in crew redistribution and resultant overcrowding. Shut-down machinery would also add its residual heat to the compartments. More so, the actual survival period in the disabled submarine could be as long as 3–7 days. All these factors indicate that the temperature and humidity profile and the resultant thermal stress inside an actual disabled submarine could be higher than what was seen in the present simulation. Thermal stress can compromise cognitive function and has been shown to cause unsafe behaviour in hot working environments.¹⁵ During the period of survival, the crew in the actual DISSUB would need to remain in a high state of alertness because the conditions inside the submarine could change adversely very quickly. In such a scenario, impaired cognition occurring as a result of thermal stress could impair physical performance, reaction time and decision-making during damage control and thus impact crew survival negatively.¹⁵ Further, increasing thermal stress could make the crew decide to escape sooner even if other conditions for survival remain favourable inside the submarine.⁸ During escape, crew with impaired hydration status could be more prone to development of decompression sickness.¹⁶ After escape too, the rescue team which would rescue survivors would need to be alert to the possibility of many of these survivors suffering from dehydration and/or heat exhaustion and would need to manage these survivors accordingly. Therefore, thermal stress could potentially affect the crew adversely in all phases of submarine escape and rescue – phase of damage control, phase of survival, during escape and even after successful escape.

Various measures can be taken to address the thermal stress onboard during the phase of survival. Generation of heat inside the submarine can be reduced by changing the regeneration system being used. The present system is highly exothermic. In comparison, the system used by German and Scandinavian submarines involves bleeding oxygen into the submarine from oxygen banks while the CO₂ is absorbed by CO₂ re-absorbent canisters fitted within the onboard ventilation system. There are some other systems as well which involve use of a re-absorbent chemical placed within porous curtains like the Lithium hydroxide curtains.⁴ All these systems are thermo-neutral and therefore could enhance thermal comfort inside the submarine. However, changing the regeneration system would require structural and engineering design changes in the submarines and therefore may not be easy to implement.

Another solution is to increase the provisioning of emergency water in the submarines. Maintaining adequate hydration is one of the primary means of preventing thermal stress.^{10, 14} The present authorisation has been in accordance with sea survival rations on life rafts. On life rafts, cooling effects of air currents prevent severe heat stress which is not the case inside a DISSUB, as the present simulation showed. However, the increase in authorisation of water has to be weighed against the space available inside the submarine to store additional emergency water. Lack of dietary salt can be addressed by replacing the present emergency rations with standardised sea survival biscuit rations which contain adequate salt.

Heat production during the survival phase could be minimised by reducing metabolic heat production by reducing body activity to an absolute minimum. The crew should therefore be resting as far as possible inside the submarine awaiting rescue in the survival phase of the DISSUB. Mitigation of effects of thermal stress can be done by limb immersion in water. Immersing the hands and forearms in cool or room temperature water has been suggested as a practical and effective method of preventing the adverse effects of thermal stress.¹⁷

Apart from these recommendations, acclimatisation to heat and aerobic fitness from long-term training are important aspects in reducing the effects of thermal stress.^{9, 10} Accordingly, physical fitness of crew particularly aerobic fitness is a measure that can reduce effects of thermal stress on crew.

This study has some limitations, first of which is that body temperature of crew was not measured. This could have been a valuable indicator of effects of thermal stress during the simulation.

Future studies could be carried out to study the temperature and humidity profile over longer periods of as much as 72 h. A study component which could be varied from the present study could be the amount of water intake. By allowing, ad libitum water throughout the simulation, effects on crew could be compared with those seen in the present study.

The present simulation was the first of its kind for the Indian Navy and showed that the crew inside a disabled submarine would be subjected to considerable thermal stress and dehydration would be a major medical problem to be tackled especially in our tropical conditions. While thermal stress cannot be eliminated, steps can be taken so that adverse effects as a result of thermal stress may be delayed and/or minimised.

Disclosure of competing interest

The authors have none to declare.